This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates in general to the art of providing refractory coatings and more particularly to a coating composition containing Y.sub.2 O.sub.3 which is effective for coating graphite ware to prevent carbon contamination during the melting and alloying of reactive metals.
In the nuclear industry, there exists an extensive need for high density shielding materials which may be easily fabricated, heat treated to a wide range of mechanical properties, and maintained chemically inert in nominally atmospheric conditions. Uranium metal is particularly useful for such applications, however, unalloyed uranium is highly susceptible to corrosion and exhibits anisotropic properties. Improved corrosion resistance may be attained by alloying uranium with various amounts of alloying metals such as molybdenum, vanadium, niobium, tungsten, and titanium. When alloyed in the proper proportions, the binary, ternary and quaternary alloys have improved corrosion resistance and possess more isotropic qualties. The improved properties are a result of the presence of a metastable gamma phase in quenched alloys. In order to control the properties of the alloy, the composition must be controlled within a very narrow range. Low temperature ageing of these gamma-stabilized uranium alloys can produce a wide range of yield strengths and elongations. For example, current studies have shown that the best combination of strength and corrosion resistance can be realized with alloys in the uranium-niobium-zirconium system and in particular with a uranium-16.6 atom percent niobium-5.6 atom zirconium alloy. It is essential that the composition of uranium alloys be controlled within a very narrow range in order to obtain the desired properties.
This requirement for strict composition control in uranium alloys has been the source of a very troublesome coating problem. Alloying metals which are reactive with carbon, such as molybdenum, niobium, zirconium, and titanium, have a strong tendency to react with graphite ware used in induction melting operations, forming carbides. These carbides float to the surface of the melt and cause the final alloy to be deficient in the reacted metals. Furthermore, uranium also can form carbides from graphite. In order to prevent this carbide formation with reactive metals, it is necessary that graphite ware (crucibles, stirrers, stoppers, etc.) be provided with a tough unreactive coating capable of withstanding the melting and alloying temperatures.
In order for a coating method to be effective for preventing carbon contamination from graphite ware during induction melting of uranium alloys, the coating must meet several requirements: it must be adherent and exhibit no cracking during application, drying, and heating; it must be sufficiently tough to withstand the impact of solid feed metals which are mechanically or manually fed into the crucible; the coating must be non-reactive with the molten metals and preferably not wet by them; and the coating must not react with the crucible and evolve contaminating gases such as CO and CO.sub.2. Aside from these quality considerations, it is highly desirable that the coating composition have a long shelf life, be applicable in a routine manner, and require no thermal cure independent of the melting heat-up.